1. Field of the Invention
This invention relates to a fixing device for fixing an unfixed image on a recording material by pressure or heat and pressure, and relates preferably to a fixing device having a pair of fixing rotatable members such as rollers or belts for fixing an unfixed toner image on a recording material.
Apparatuses to which the present invention is applicable include image forming apparatuses such as printers, facsimiles, copying apparatuses and printing machines, or various devices for fixing unfixed images on recording materials.
2. Description of the Prior Art
Fixing devices using a pair of rotatable members have heretofore often been used and above all, the roller fixation is most frequently used. This fixation is represented by heating-pressing fixation and pressure fixation.
The roller fixing devices may generally be divided into the following combinations (1) and (2): (1) a system using as a fixing roller for contacting a toner image including a rigid roller having a metal core coated with so-called Teflon such as PTFE or PFA and using as a pressure roller a roller coated with an elastic material such as silicon rubber, fluorine rubber, EPDM, Hydrin rubber or fluorosilicon rubber, and (2) a system using rollers coated with an elastic material as a fixing roller and a pressure roller.
Of the above-mentioned systems (1) and (2), where the roller coated with an elastic material mentioned under item (2) is used as the fixing roller, as compared with a case where the roller coated with Teflon is used as the fixing, roller, the device is excellent in fixing ability, image-producing properties and curling-preventing properties (and is particularly effective for automatic two side copying aparatuses) and therefore is very useful.
However, in the case of the system (2), if the surface of the fixing roller is rough, the roller is liable to be contaminated by toner during long-time use, and there has been a disadvantage that it is very difficult to remove the toner which has once adhered to the surface of the roller. Also, once the toner adheres to the surface of the roller, toner offset sharply increases because of the good affinity between such toner and the toner on copy paper, and this has caused jamming and other various troubles.
Also, a fixing roller equipped with a heater therein is usually weak in strength because the thickness of the elastic material must unavoidably be made small to promote heat conduction. When a cleaning member such as a felt, a web or a blade cleaning roller is caused to bear against the fixing roller, the elastic material may be damaged or other problems in respect of its durability may occur.
On the other hand, if the surface of the roller is made smooth to eliminate the above-noted inconveniences, the electrostatic attraction and vacuum attraction of copy paper to the roller increase and separability of the copy paper from the roller becomes more difficult.
Therefore, it becomes necessary to take a measure such as causing a pawl-like separating member to bear against the roller with a strong pressure, but this may give rise to a problem such as damaging the roller.